How does regular exercise protect against mental decline?

Exercise is a powerful tool to help protect the brain against the detrimental effects of aging. It
•    rushes oxygenated blood to nourish brain cells and fosters the growth of new blood vessels in the brain 
•    increases levels of brain-derived neurotrophic factor, a molecule that encourages the growth of new neurons and maintains the health of existing ones
•    preserves and even increases volume in regions of the brain that tend to shrink with age, such as the prefrontal areas and hippocampus
•    maintains the integrity of white matter, which is fundamental for the transmission of information within the brain
•    enhances cognitive control and reduces impulsivity, promoting better decision making
•    improves function of the brain networks that promote sleep